Traffic Collision Avoidance Systems (TCAS) use two antennas, one on the top and one on the bottom of an aircraft to estimate the relative bearing between the aircraft and an intruder (e.g. another aircraft). Each traditional TCAS antenna has four elements, which are placed orthogonally on the same plane. A TCAS system interrogates the transponders of other aircraft. The transponders of other aircraft respond with a reply which may contain altitude or other information. The TCAS system may also use the reply signal to estimate the relative bearing of the other aircraft.